In recent years, an apparatus which transmits power in a noncontact manner has widely spread. A power transmission apparatus includes a power transmission device which transmits power, and a power reception device which receives the transmitted power. The power transmission apparatus transmits power from the power transmission device to the power reception device in a noncontact manner by using an electromagnetic induction system, a magnetic field resonance system, an electric field coupling method, or the like. The power reception device includes a driving circuit which drives the power reception device, and a load circuit such as a charging circuit of a secondary battery mounted in the power reception device.
When power (power of up to about several tens of watts) is transmitted to an electronic apparatus such as a portable terminal or a notebook PC in a noncontact manner, if the electromagnetic induction system or the electric field coupling method is used, the power transmission device and the power reception device are generally required to be substantially brought into close contact with each other in a transmittable region. On the other hand, if the magnetic field resonance system is used, the power transmission device and the power reception device are not required to be brought into close contact with each other, and, for example, even if the power reception device is separated from the power transmission device by about several cm, power can be transmitted. Therefore, the magnetic field resonance system has attracted attention since there is a degree of freedom of a position where the power reception device is placed, and convenience is excellent.
In the magnetic field resonance system, a resonance element formed by a coil and a capacitor provided in a power transmission device and a resonance element formed by a coil and a capacitor provided in a power reception device are coupled with each other, and thus power can be transmitted. Even in the electromagnetic induction system, a trial has been conducted to increase a power transmission distance not only by coupling a coil of a power transmission side and a coil of a power reception side with each other but also by providing resonance capacitors in both the power transmission side and the power reception side so as to resonantly couple elements of the power transmission side and the power reception side with each other. Therefore, differentiation between the magnetic field resonance system and the electromagnetic induction system has disappeared.
In addition, as a parameter which influences power transmission efficiency, there is a coupling coefficient k between resonance elements of a power transmission device and a power reception device. If a distance between the resonance elements of the power transmission device and the power reception device varies, the coupling coefficient k also typically varies. For example, if a distance between the resonance elements increases, the coupling coefficient k decreases. If the impedance of a circuit is fixed, the power transmission efficiency varies according to a variation in the coupling coefficient k.
As a method of maintaining the power transmission efficiency to be high even if the coupling coefficient k varies according to a variation in a distance between the resonance elements of the power transmission device and the power reception device, there is a technique in which an impedance adjusting unit which can vary impedance is provided, and impedance of the power transmission device or the power reception device is varied according to a variation in the coupling coefficient k (refer to JP-A-2011-50140).
However, in the technique disclosed in JP-A-2011-50140, there is a problem in that a new circuit for automatically controlling impedance is necessary when the coupling coefficient k varies, and control is complex.